One of the deepest and most central problems in neuroscience is how neurons and circuits manage to balance the need for plasticity with the requirements of neuronal and circuit stability. Pioneering work over the past two decades from the investigators on this program project has established that neurons and circuits solve this stability problem by engaging a set of homeostatic plasticity mechanisms that act to stabilize neuronal excitability around an activity set-point. The study of homeostatic plasticity has now blossomed into a large and diverse field, and a substantial body of literature describing the phenomenology of homeostatic regulation of neuronal excitability and synaptic strength now exists. However, despite much progress, key elements of the conceptual framework for understanding homeostatic regulation of neuronal activity remain completely obscure. In this proposal, we wish to address four outstanding questions in the field: 1) How are neuronal activity set-points built, and what are the molecular identities of the set-point components? 2) How are different set-points in different neurons established? 3) What are the downstream targets of the set-point machinery, and how are these organized to effectively coordinate different forms of homeostatic plasticity? And finally 4), how do network relationships influence set-points? This program project will bring together four of the top researchers in the field of homeostatic plasticity in a synergistic enterprise to determine how activity set-points are built. Our premise is that this process is so fundamental to all nervous systems that it is likely that th functional structures underlying it are evolutionarily conserved. Therefore, we now propose a collaborative and synergistic set of experiments that will leverage the advantages of different vertebrate and invertebrate preparations to uncover the molecular identity of the sensors and effectors that regulate homeostasis of activity set-points. The experiments described here will test novel hypotheses about how a truly fundamental problem in neuroscience is solved, and in the long term are likely to result in a set of new strategies for interventions in numerous neurological disorders that result from impaired homeostatic and compensatory mechanisms.